The heroin addict almost always injects the drug directly into a vein, which is known as "mainlining." In this way the junkie gets the desired effect of a sudden rush and high, which comes when the drug enters directly into the blood stream.

The addict does not use sterile needles, and the heroin and the water used to dissolve the heroin are not sterile either. For these reasons, there is always infection at the injection site. After a period of time, scars or "needle tracks" form along the infected veins. It is not uncommon for the addict to try to hide his needle tracks by covering them with tattoos.